Originally Posted by Tourdog
The clutch rod spins whether engaged or disengaged. But, thinking it thru with a mechanically (but adjustable) actuated clutch vs the hydraulically actuated that is "not adjustable" but fixed by virtue of engineered position, throw, and rod length yields little difference. But, since the clutch rod dead centers on the diaphragm spring and therefore, no radius (for torque) it then has the possibility of an infinitely tiny point which produces zero rotation. A dab of required grease at the pointed end (to take care of the non-infinite) and a larger dab at the slave thrust end will reduce compression friction especially when the clutch lever is relaxed (ie engaged) and IMO it will rotate little. It is those that do rotate that ruin the slave bearing (spins the piston) and scour the slave walls and R & R is required. I venture that most who do the "slave", R & R, never pull the slave apart so they don't know if it spun or not. If much rotation was expected by BMW engineering that slave bearing would be more robust than it is.
The forward end of the shaft goes through the clutch diaphragm spring and into the engine output shaft. The part that rests in the center of the diapraghm is as large as what rests against the piston bearing, but chamfered, so actually more surface area. The rod never spins against the diapragm spring, even if lubricated. I put a little lube on mine, but next time it was removed there was the normal expected brown "fretting" on the forward end, no rotational movement evident.
When the clutch is engaged (lever out) the piston is pushed back against the piston spring, so the spring is loaded at it's heaviest. That is a pretty decent load on the little bearing.
Yes, I am very surprised that BMW designed in such a small bearing that runs constantly. Unfortunately, there is little room available to make something bigger. The swing arm is right behind the slave cylinder, and the transmission is already remarkably thin for all that is in it. If I were to re-design it, I would change the design of the diapragm spring to allow a much larger bearing to be installed in it (there is room) and add a better pilot bearing in the end of the engine output shaft or eliminate that guide end of the actuation rod. A feature could be added to prevent rod rotation, but then it would still be inside the transmission input shaft for much of it's length, with that spinning when the bike is in motion.
That little bearing in the slave piston is one overworked little dude! Amazing most last as long as they do.
I stongly recommend periodic Preventive Maintenance replacement of the slave! Maximum 60,000 miles, maybe 48,000 or 54,000 service intervals would be safer. Even as much as I rode that would still only be every 18 months to 2 years. A lot longer for most. Shorter for a few.